1. Field of the Invention
The present invention relates to a protective system for a machine tool, e.g., for a circular saw, which system affords its protective effect within a very short time span that is usually in the range of a few milliseconds, in order to protect a user of the machine tool from injury in hazardous situations.
2. Description of Related Art
There exist at present substantially three different approaches to implementing a protective system of this kind for table saws and format circular saws, which is intended to prevent a user from coming into contact with the revolving saw blade or suffering a serious cut injury therefrom.
The protective system, offered and marketed under the company name Saw Stop Inc., of an American manufacturer involves an emergency brake system that enables braking of the machine tool on the basis of direct engagement of a brake actuation system with the saw blade as soon as a correspondingly configured sensor has detected a hazardous situation. By the use of a heated-wire trigger, a rotatably disposed aluminum block is pushed with the aid of a preloaded spring into the teeth of the running saw blade, which wedges itself thereinto and in that fashion absorbs the rotation energy of all the machine tool geometries that revolve during the sawing operation. As a side effect, this one-sided application of force onto the cutting edges is used to recess the saw blade into the saw table by way of a specially designed suspension system disposed on the saw table. This assemblage makes it possible to avert serious bodily damage to the operator of the machine tool who triggers the protective mechanism. A disadvantage in this context is the direct action on the machining tool, i.e. the cutting geometry of the saw blade, since an additional hazard potential for the user arises because parts of the tooth set break away. In addition, restoring the readiness of the protective system requires exchanging the brake unit, including the saw blade, for a replacement unit that is ready for use and that the user must keep on hand in order to continue working after a braking operation involving utilization of the protective function has occurred. This is associated with considerable consequential costs and a corresponding outlay of time for acquisition and installation. It can furthermore be assumed that all the components affected by the braking operation, i.e. all the revolving geometries of the machine tool, will be exposed to large stresses during the deceleration phase. Neither the manufacturer nor corresponding publications about this system provide data regarding the long-term durability of the system.
A further approach involves use of a protective system exclusively for lowering the saw blade into the saw table, without initiating a saw blade braking operation. Here the saw blade, including the main shaft and its bearing system, is removed from the hazard zone with the aid of a pyrotechnic initiating charge, so that serious injuries to the user can be prevented. This type of protective system is disadvantageous in that it requires relatively large masses, in the form of the assemblages to be lowered, to be moved under strict time constraints on the order of milliseconds. The pyrotechnic initiators necessary as a result, which turn out to be indispensable for this protective actuation suite, moreover produce an expensive partial reversibility that limits, in terms of both time and organization, immediate continuation of work on the machine tool with a protective system that is ready to use. In addition, for reasons of limited installation space and a very specific procedure, this protective system is suitable only for larger stationary units, for example table saws, which allow this kind of construction in their interior. This system is, however, not qualified for use on smaller manually guided units, for example cutoff and miter saws or panel saws.
A publication from a publicly funded project called “Cut-Stop” (VDI/VDE/IT) regarding a protective system for format saws from the Institute for Machine Tools (IFW) of the University of Stuttgart describes an approach that brings the main shaft of the machine tool, and thus the saw blade, to a standstill using a special form of a disk brake system, namely a self-reinforcing wedge brake. A wedge is accelerated with the aid of a pyrotechnic initiator and then pushed between a stationary wedge guide in the shape of a modified brake caliper, and the rotating brake disk. The assemblage acts in self-locking fashion for a specific selection and combination of wedge angle α and brake lining values μ, so that the particular time-related requirements for the braking operation, depending on the inertia to be decelerated, can be met with this configuration. The disadvantage of this protective system, however, is that here as well, as described in the publication cited, a replacement of the entire brake unit is necessary subsequent to triggering of the protective actuator system, because of the wedge that jams into the friction pair. The time required to perform the work involved is approximately 10 to 12 minutes for complete system readiness to be restored. A partial limitation of immediate system reversibility therefore exists here as well.
Published German patent document DE 195 36 995 A1 discloses a safety brake for elevators which has a device that, when the transport means exceeds a predefined maximum velocity, brakes (and optionally also arrests) the drive system with a velocity-related deceleration. The safety brake of published German patent document DE 195 36 995 A1 acts in rotation-speed-dependent fashion directly on the traction sheave of the cable-operated conveying system, and allows its rotation speed to be limited. The safety brake, embodied as a centrifugal brake, additionally has a device for amplifying the braking force as a function of the conveying speed.
Proceeding from this existing art, it is an object of the present invention to create a protective system of a machine tool having an alternative configuration, which system affords its protective effect within a few milliseconds and at least partly eliminates the problems described above.